Rotary drilling of deep wells for the production of fluid minerals, such as oil and gas, relies upon long assemblies of pipe called “strings.” Each separate pipe unit or section for this purpose normally is in the order of 9 to 12 meters (30 to 40 feet) in length and threaded at each end.
Drill pipe, which forms the primary pipe string for advancing the bore hole depth and often provides rotational torque to the drill bit, is usually fabricated with tapered external threads at one end and tapered internal threads at the opposite end. Drill pipe external threads are formed into a heavy tool joint called a “pin” that is welded to the one end of a pipe section. Internal drill pipe threads are formed into a complementary tool joint called a “box” that is welded to the opposite pipe end.
“Oil field” casing and tubing pipe are usually formed with external threads at both ends of a pipe section. Two sections of pipe can be joined together by a short length (close) coupling having internal threads at opposite ends.
In the course of downhole operations, such pipe strings occasionally become tightly stuck in a well. Typically, the bore hole walls of a loose or unstable geological strata, penetrated by the drill string, “sluffs” or collapses into the borehole around the drill string and above the bit. Such a wall collapse may occur for hundreds or even thousands of feet along the borehole length. In such an event, it is impossible to withdraw the drill string from the borehole or, in most cases, even rotate the drill string.
Often, it is desirable to retrieve as much of the pipe string above the seizure point as possible. In any case, it is essential to extract the drill string above the seizure point to enable further operations. However, simply reversing the rotation of the pipe string will not necessarily separate the string at the first threaded joint above the seizure. As additional pipe sections are added to a string, the earlier assembled joints become tighter and more difficult to unthread and separate. Consequently, without some focused intervention, an upper threaded joint will normally disassemble before a lower joint.
There are numerous existing methods and devices for locating the seizure point in a pipe string. The method and apparatus taught by U.S. Pat. No. 7,383,876 is representative of existing technology. After locating the specific joint above the seizure point, the traditional method used to effect release of the threaded assembly at that specific joint is to apply a gentle or moderate “left hand” torque to the pipe string, as the specific joint is shocked or “jarred” by a nearby explosion.
Explosive devices for urging the release of threaded joints, which are joined together, have heretofore been made in various forms. Typically, a “back-off tool”, as such devices are characterized in the well drilling arts, comprises detonation cord, such as “Primacord”, which is a flexible tube filled with a suitable high explosive that is set off by an electrically initiated detonator. When used under low temperature and pressure conditions, prior art “back-off” tools and methods have produced generally satisfactory results. However, in extremely deep wells, temperatures are in the order of 200° C. or greater, and the pressures are several thousand pounds per square inch, thereby presenting the prior art apparatus and methods with serious functional and reliability issues.
A need exists for a back-off tool that is usable and reliable in deep well environments, which include exposure to fluids and increased wellbore pressures, for the unthreading (e.g., unscrewing, decoupling) of joints of tubulars (e.g., drill pipe, casing).
A need exists for a back-off tool that is usable and reliable in deep well environments where high pressures and high temperatures within the wellbore result in difficult explosive transfers between detonators and explosives, and especially where such back-off tools are configured to utilize the ambient pressure to facilitate and advantage the detonation characteristics.
The present invention meets this need.